This is interesting, if it's truly practical (Kamen's brilliant, but sometimes has an unorthodox view of "practical"). Stirling engines have traditionally been

This is interesting, if it's truly practical (Kamen's brilliant, but sometimes has an unorthodox view of "practical"). Stirling engines have traditionally been more efficient, but also more expensive(1) and much heavier(2) than competing technologies. The second factor, especially, impacts automotive applications. (1) http://www.gulfcoastchp.org/WhatIsCHP/Technologies/Power/Stirling http://www.gulfcoastchp.org/WhatIsCHP/Technologies/Power/Sti... (double the $/kw of ICE) (2) http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APCPCS000880000001000466000001&idtype=cvips&gifs=yes http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=n... (even NASA only hopes to develop a 77lb, ~7hp Stirling, not the greatest power to weight ratio).